There are some differences in the splines on the pinion 6/8 I think but shouldn't come into play on your car. There was a really good article about those rear ends a year or so ago in either Rod & Custom or Old School Rods. I'll try to find it . My dad & I shortened a '48 Chevy closed drive 2 1/2". Like you say the outside is a snap. We cut the shaft and beveled the edges. Then dad tacked it together and rolled it across the floor to check for straightness. Then he welded more and rolled more till we thought it was straight and put it together. I drove it for years behind a good running 235. Two other guys I know owned it after me . Never a problem.
